People conveyors such as escalators or moving walkways which are formed from adjacent moving steps include a passenger carrying path of travel, which begins and ends at opposed landings, and a return path of travel disposed beneath the passenger carrying path of travel and out of sight of passengers. The sprockets engage and guide step chains through a 180.degree. arc to reverse the direction of step movement. As the steps pass over the sprockets, the steps invert and re-invert their spatial orientation.
With extensive usage and equipment aging, the possibility arises that a step may break loose from the step chain. If a step thus should break loose, it will swing by gravity away from its normal path of travel and the step tread will fall downwardly. When the steps are properly connected together on the step chain, there will be a constant procession of steps past any given point along the path of travel, and there will not exist any significant gaps in the step procession. When a step breaks loose, a significant gap will be created in the procession of steps. Further, the conveyor drive may continue to operate so that a person using the conveyor would not know that a step is missing or out of place. This could result in injury to passengers when the displaced step returns to the passenger-carrying path of travel.
The problem of detecting abnormally positioned passenger conveyor steps has been addressed in the prior art. One prior art system discloses a monitor for an escalator for detecting the presence or absence of the escalator step rollers to detect detached escalator steps, should one occur. This mechanical arrangement is expensive. A second prior art system shows an inductive proximity sensor at a step and if the inductive proximity sensor detects no step for a time greater than a time limit stored in a timer, then a missing step signal is provided and the escalator stopped. A disadvantage of this system is the cost of the timer. A second disadvantage is that for a fully loaded escalator or an older escalator with deteriorated performance, the escalator moves more slowly than otherwise and the detection of the normal gap between steps may be mistaken for a missing step. Third, the timer requires fine calibration so that the time intervals stored in the timer correspond exactly with the time for a step and the gap between two steps to pass the inductive proximity sensor. Or, if for some reason the escalator is moving excessively fast, a step may be missing but go undetected, resulting in harm to any passenger stepping into the consequent void. A third prior art system discloses an escalator step which uses photoelectric detectors below the steps to detect the dropping of a step. This system also requires a timer.
A fourth system discloses a mechanical sensor placed beside the return run of the steps on an escalator or moving walk. The sensor is biased toward the step so as to bear against each step passing thereby. If a step in the series is missing from its normal position, the sensor moves in the direction of the step run and opens a switch, thereby shutting off power to the escalator.
In sum, all of the above schemes detect a missing step by sensing a single step and using a timer, or by being actuated by a single step.